[ please, when you experience a problem and are able to identify the commit that triggers it, don't forget to CC the author of the commit in question -- me, in this case ]

As far as I remembers, Ubuntu uses klibc in initramfs, right?

What version of klibc do you have? There was a bug in klibc that causes such behavior on PIE-randomization-enabled kernels, and has been fixed in klibc-1.45 by commit [1]. Please make sure you have updated klibc version that doesn't contain this bug.